Braking systems for motor vehicles having two independent (separate) hydraulic brake circuits are known from the related art. Due to the independent design of the two brake circuits, in the event of a circuit failure due to a brake circuit defect it should still be possible to ensure at least minimum functioning of the braking system, and thus prevent a total failure of the braking system.
A method for ascertaining a malfunction of a pressure sensor or a brake circuit of a hydraulic vehicle braking system having two brake circuits is described in German Published Patent Application No. 102 24 590, for example. The malfunction or the failure is recognizable by comparing a hydraulic pressure ascertained with the aid of the pressure sensor in a pressure chamber of a main brake cylinder (or a variable derived therefrom) to a vacuum pressure of at least one vacuum sensor situated at a vacuum brake booster (or a variable derived therefrom). After the malfunction or failure is recognized, a shutoff valve situated in a hydraulic line between the main brake cylinder and the still functional brake circuit is closed, and at least one wheel brake of the still functional brake circuit is subsequently acted on by pressure with the aid of a hydraulic pump. In this way, the aim is for hydraulic brake boosting to be producible despite the malfunction or the failure.